Split up the compiler DAG

[ferd]

This is another tricky commit towards replacing the current module
analysis with EPP. The compiler DAG is now being shared across multiple
applications being compiled, rather than a per-application basis, which
promises to allow better ordering, parallelism, and more thorough
invalidation of runtime dependencies when they are modified.

This however required changes:

• The compiler DAG is no longer private to rebar_compiler, and has
  been extracted to the rebar_compiler_dag module
• The compiler DAG is now started by the rebar_prv_compile module,
  which oversees the calls to rebar_compiler for each OTP application
• The compiler DAG has been refactored to use a "dirty flag" to know if
  it was modified, rather than just tracking modifications in a
  functional manner, since the scope change (going multi-app) makes it
  impossible to cleanly use the functional approach without much larger
  changes
• The DAG used to be cached within each OTP application. This is no
  longer possible since it is shared. Instead the DAG is stored in the
  state's deps_dir, which allows to cleanly split caches between regular
  apps for the user's project and plugins
• The DAG supported a "label" mode that was used to store distinct DAGs
  for extra_src_dir runs and regular modules; this label is now used
  (and extended to rebar_prv_compile internals) to distinguish between
  "compile runs", such as "project_apps", or just "apps" (deps). The
  label is optional (i.e. not used by plugins which have no such need)
• The extra_src_dirs for each app is now compiled using the main app's
  DAG, but the run takes place later in the compilation process. This
  may need changing to detect and prevent dependencies from src_dirs
  into extra_src_dirs, but this should not technically be a problem for
  runtime anyway.
• Reworked the support for extra_src_dirs that are at the root of an
  umbrella project (and therefore do not belong to any single app) to
  use the new structure, also as part of the project_apps DAG.
• The clean command no longer clears the DAG since it's not part of
  the app itself anymore; the prune step should handle it.
• I dropped the "include dir changes deletes the DAG" criteria since it
  clearly doesn't make sense anymore. I plan to eventually replace it with
  things such as the compiler version instead.

All tests keep passing, and this puts us in a better place to use EPP
with cross-app support in the near-future.

[tsloughter]

Can we really merge this without this disabled by default?

[ferd]

yeah it's what we have today. See the complaints about modules auto-compiled by CT. I just added the note in comments to fix this long-standing bug.

[tsloughter]

We should decide on how many versions back of rebar3 to support and figure out a way to give useful error messages when plugins are outdated.

Doesn't need to be a part of this PR, just bringing it up because this is adding to the already extensive technical debt for plugin support.

[ferd]

yeah. To some extent I have never taken the time to update the rebar3_proper plugin myself so I feel in a bad place to force others to do it as well. It still displays the cool warning every time you run it.